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Article

Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp

by
Ana Alves
*,
José Graça
and
José Rodrigues
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Tapada da Ajuda, 1349-017 Lisboa, Portugal
*
Author to whom correspondence should be addressed.
Forests 2022, 13(12), 2169; https://doi.org/10.3390/f13122169
Submission received: 17 November 2022 / Revised: 14 December 2022 / Accepted: 15 December 2022 / Published: 17 December 2022
(This article belongs to the Special Issue Lignin: The Hidden Forest Product)
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Abstract

:
The residual lignin content of unbleached maritime pine (Pinus pinaster Aiton) Kraft pulps was assessed by analytical pyrolysis (Py-lignin) and the results were compared to the Klason lignin content and kappa number. Thirty samples, each from an individual tree, were delignified under identical conditions. The residual lignin content of the pulps varied widely as assessed by Py-lignin (5.9%–9.2%), Klason (8.2–15.1), and kappa number (59–112). Despite a systematic difference between Py-lignin and Klason, they were strongly correlated (R2 = 0.90). The H/G ratio of the residual pulp lignin ranged from 0.145 to 0.195, with a mean of 0.165, which is more than two times the average H/G ratio of Maritime pine wood lignin (0.064). The results show that Kraft pulping, which selectively degrades lignin with more labile inter-unit links, changed the pattern of pyrolysis products of pulp lignin considerably and, consequently, its structure. This pattern shows an enrichment in H-lignin-derived products, namely phenol, p-cresol, and m-cresol, and in some G-lignin-derived products such as guaiacol and 4-mthylguaiacol, and a decrease in coniferylaldehyde, homovanillin, and eugenol. Principal component analysis (PCA) of the G- and H-lignin-derived pyrolysis products shows that pulps are distributed along PC 1 based on their residual lignin content. The loadings plot shows that this separation is mainly due to a small number of G-lignin products, including 4-methyl guaiacol, 4-vinyl guaiacol, isoeugenol (trans), and guaiacol, which are more abundant in pulps with higher residual lignin content. The obtained results show that analytical pyrolysis is an appropriate method for quantifying the residual lignin content and H/G ratio of unbleached Kraft pulps and provide information regarding how lignin is degraded during the pulping process.

1. Introduction

Maritime pine (Pinus pinaster Ait.) is a species native to Portugal, Spain, Southern France, Corsica, Sardinia, Morocco, and Tunisia [1]. The unbleached Kraft pulps are mainly used for packaging paper and paperboard manufacturing [2,3,4]. In the industry, the residual lignin in pulp is determined by the kappa number, an indirect method of measuring the pulpwood quality and an indicator of the degree of delignification and bleachability [2,3,4,5]. The chemical characterization of wood and different types of pulps using traditional analytical techniques is normally a tedious procedure. On the contrary, analytical pyrolysis is a fast and reproducible method with simple sample preparation (drying and milling) that requires very small sample sizes (70–90 µg range).
Analytical pyrolysis has become a standard method for determining H/G/S lignin ratios in many materials, with a vast amount of literature supporting its efficacy (reviewed in [6]). This technique is able to assess the lignin content, even in cellulosic matrixes with minimal lignin amounts [7,8,9,10,11,12,13,14,15,16,17,18,19,20]. However, only a few works have dealt with measuring small differences in lignin content or composition, namely the intra-species tree-to-tree minor variation [7,8,9,10,11,21,22], usually under strong genetic control and important for tree improvement programs [4,23,24,25,26].
The precision of the Py-lignin determinations (0.37–0.41%) is good and close to the precision of the Klason method (0.34%) [7,10,27]. However, a systematic bias exists between Py-lignin determinations and the lignin content as assessed by Klason or total lignin (Klason+ acid soluble lignin). In Eucalyptus, the Py-lignin overestimates the Klason lignin, and this bias is close to the acid-soluble lignin value, so the total lignin content is similar to Py-lignin, such as E. globulus (0.8% less than total lignin), E. dunnii (0.4%), E. camaldulensis and E. grandis (1.9%), and E. tereticornis (1.6%) [10]. In the case of softwoods, Py-lignin underestimates Klason lignin (3.9%) for Pinus pinaster, Picea abies, and Larch spp. [7,27]. These differences between hardwoods and softwoods could be explained by the differences in the frequency of the most labile inter-unit β-O-4 linkages, accounting for 60–65% of the bonds in hardwood lignins and half of that in softwood lignins [28]. Softwood lignins are more condensed, with more C-C bonds, which are harder to degrade even in analytical pyrolysis conditions. Nevertheless, due to good correlations between Py-lignin and Klason (or total) lignin, the lignin content can be accurately estimated from Py-lignin.
Although analytical pyrolysis, due to the extensive degradation it imparts in lignin, does not provide direct information on inter-unit linkages, different patterns of pyrolysis products can indirectly provide structural information on lignin [19,29].
The goal of this work was to investigate the ability of analytical pyrolysis to quantify the amount of remaining lignin and the differences in the H/G ratio between pulps obtained from the wood of different trees under identical pulping conditions and to evaluate eventual structural differences in the pulp’s residual lignins.

2. Material and Methods

Thirty maritime pine Kraft pulp samples, obtained from the wood of trees prepared under identical cooking conditions (active alkali charge of 18.5% (as Na2O), sulfidity 30%, H-factor 1600), had kappa numbers ranging from 59.3 to 102.5, with an average of 82.3 and a standard deviation of 12.3 (details of the total number of samples analyzed can be found elsewhere [4].

2.1. Analytical Pyrolysis (Py-GC/FID)

The pulps were first dried in an oven at 60 °C overnight and after were milled in a Thomas mill with a 0.12 mm sieve. Pyrolysis was performed with a CDS Pyroprobe 1000 equipped with a coil filament probe. The pyrolizer was connected to a Hewlett Packard HP 5890 Series II gas chromatograph by a heated interface (270 °C) and a flame ionization detector (FID). About 75–80 μg of pulp were weighed exactly on a quartz boat and pyrolyzed at 650 °C for 10 s with a temperature rise time of approximately 20.0 °C ms−1. The GC is fitted with a fused-silica capillary column DB1701 (60 m × 0.25 mm, 0.25 μm film thickness, J&W Scientific). The injector and detector were kept at 270 °C, and the oven temperature was held at 45 °C for 4 min and raised to 270 °C at 4 °C min−1. The pyrolysis products of selected pulps and milled wood lignin (MWL) were analyzed by Py-GC/MS (CDS Pyroprobe 1000 connected to a HP 6890 with a HP 5973 Mass Selective Detector) for identification of pyrolysis products based on their mass spectra and retention times by comparison with the NIST library and with the literature [7,8,9,17,22,27]. In addition to retention times, a Pinus pinaster MWL was used as a standard to help with the synchronization of the peaks between MSD and FID pyrograms. At least two pyrolysis runs (Py-GC/FID) were performed per pulp, always on different days.

2.2. Data Treatment

Peak areas of all identified products were normalized by the sum area of all identified peaks and multiplied by 100, and used for Py-lignin and H/G ratio assessment using Chemstation Software (Agilent Technologies, Palo Alto, CA, USA). The precision of the pyrolysis method was assessed by 6 replicates of the same sample on different days over 6 weeks and by the pooled standard deviation of all replicates.
The UnscramblerTM 10.4 (Camo) was used to perform Principal Component Analysis (PCA) of the lignin pyrolysis products. Prior to PCA, the replicates for each sample were averaged and the percentage of each peak was normalized (the area of a peak divided by the sum of the area of all used peaks multiplied by 100%).

2.3. Lignin Content Determination

The Klason lignin content of the pulps was determined using 500 mg of pulp using the sulphuric acid method following the procedure of Schwanninger and Hinterstoisser [30]. The kappa number was calculated by dividing the Klason lignin content by 0.13 [31] and 0.15 [19], and predicted by an existing NIR model that uses the minimum–maximum normalized spectra in the wave number range from 6110 to 5440 cm–1, a root mean square error of prediction of 2.3 units of the Kappa number, and a coefficient of determination of 95.9% [32].

3. Results and Discussion

3.1. Klason Lignin in Pulp Can Be Estimated from Py-Lignin

The predicted Kappa number of the 30 samples ranged from 82–102 with an average of 82. The correlation between the predicted Kappa number and the calculated Kappa numbers, based on the Klason results, was excellent with a very high coefficient of determination (0.97) irrespective of the correction factor used (Klason/0.13 or Klason/0.15). However, the kappa number calculated based on 0.15 gave a closer estimation (55–101) to the predicted ones than the one based on 0.13 (63–116). The Py-lignin ranged from 5.9 to 9.2%, with a mean of 7.7% and a standard deviation of 1.0%, while the Klason lignin content ranged from 8.2 to 15.1%, with an average of 11.9 and a standard deviation of 2.0. On average, Py-lignin underestimates the residual Klason lignin content by 4.3%, with the difference increasing with Klason lignin content (Figure 1). This is in agreement with earlier findings that Py-lignin underestimates the Klason lignin content of maritime pine wood by 3.9% and that the difference also increases with the Klason lignin content [7]. The underestimation of the lignin content of Kraft pulps by pyrolysis suggests that the residual lignin in the pulp is more condensed than the native lignin in the wood, since it is known that the more condensed the lignin, the fewer lignin-derived products are produced [7,8,11,33]. For Eucalyptus globulus wood, which has a less condensed lignin structure, Py-lignin overestimates the Klason lignin content and is close to the total lignin content [10,11].
Nevertheless, a high coefficient of determination (R2 = 0.90) was obtained between the Py-lignin and Klason lignin content (Figure 2), allowing the calculation of the Klason lignin content with high accuracy. A similar correlation (Py-lignin vs. Klason) with an identical coefficient of determination (R2 = 0.90) was obtained for woods (maritime pine and spruce wood samples [7] and to slightly below the coefficient of determination (R2 = 0.93) obtained with a combination of samples of maritime pine, spruce, and larch [8]. The correlation between Py-lignin and the kappa number gave a similarly high coefficient of determination (R2 = 0.90). However, the coefficient of determination for the Klason lignin content against the predicted Kappa number was higher (R2 = 0.97), indicating that the error of the pyrolysis determination is somehow higher than the error of the Klason lignin determination. This is supported by the pyrolysis precision analysis (0.48) based on six repetitions of a single sample, although the precision assessed by the pooled standard deviation of all replicates was only 0.38, suggesting that the pooled standard deviation may give estimates that are too optimistic. These results are within the reported precision of Py-lignin for maritime pine wood (0.41) and above the repeatability (0.34%) of the Klason lignin method [31,34]. Analytical pyrolysis (Py-lignin) can be used directly to scale samples based on their lignin content when the absolute amount of lignin is not essential for comparative sample analysis, or it can be used to correctly quantify the actual lignin content.

3.2. Pulp Residual Lignin and Wood Have Different H/G Ratios

The average H/G ratio of the pulps was 0.165 (0.145–0.195). This mean value is 2.6 above the 0.064 H/G ratio for maritime pine wood (0.041–0.111) [22]. This result suggests that, in the Kraft process, the G-type lignin is preferentially removed in comparison with the H-type, although a negligible and non-significant linear correlation was found for the H/G ratio against the residual Klason lignin content (R2 = 0.04; p-value = 0.30).
Ohra-aho et al. [19] also found an increase in the H/G ratio, assessed by analytical pyrolysis, from softwood wood to unbleached Kraft pulp, further increasing along the bleaching steps. To explain this increase, a number of other sources of phenol, other than lignin, the pyrolysis product formed from lignin H-moieties, are mentioned as possible; namely, polysaccharides [35,36], phenolic extractives [37], protein, and amino acids [35]. However, condensed tannins [37] and proteins [12,35] are unlikely sources of phenol due to their very low content in maritime pine wood. In the works where polysaccharides were mentioned as putative sources of phenol [35,36], the purity of cellulose was not assessed, and it is known that cellulose preparations are prone to have residual lignin, as explained by Fengel and Wegener [38]. Furthermore, the pyrolysis of maritime pine alpha-cellulose produced phenol but in much lower amounts than guaiacol (unpublished results), showing that both were probably products of the residual lignin present. However, in our pyrolysis conditions, the phenol content generated, if any, from cellulose would be too low to account for the increased H/G ratio.
The observed doubling of the H/G ratio in pulp as compared to wood means that, during the pulping process, about three-quarters of the lignin G-units were lost but less than half of the H-units (discussed below).

3.3. Pulp Residual Lignin and Wood Have Different Patterns of Pyrolysis Products

Table 1 shows the lignin-derived products based on total pyrolysis products (a) and lignin pyrolysis products only (b), meaning the sum of lignin pyrolysis products equals 100. The sum of the pyrolysis products wood (a) and pulp (a) is 24.0% and 7.3%, respectively, corresponding to the average Py-lignin of the wood (23 samples) and of the pulps (30 samples). As can be seen, the calculations are based on all products (a) the lignin pyrolysis products of the pulp are reduced compared to the wood. However, when calculated based on lignin products (b), it is easy to spot the pyrolysis products whose relative quantities have changed. Besides H-type products (H 1, H 2, and H 3), guaiacol (G 1) 3- methyl guaiacol (G 3), isoeugenol (G 8), and dihydroconiferyl alcohol (G 19) also had a relative increase. Among others, eugenol (G 5), homovanillin (G 12), the isomer of coniferyl alcohol (G 16), and coniferylaldehyde (G 22) were less represented in the pyrolysis products of the unbleached Kraft pulp. This supports the hypothesis that the H/G ratio increase was due mainly to the differential degradation of G-type lignin.
The comparison of the relative yield of pyrolysis products from the pulp versus wood could also give a hint about the origin of the products; for instance, guaiacol (G 1) is thought to be mainly produced by the direct breaking of β-O-4 bonds [39,40] or of 5-5 condensation linkages [41]. The fact that delignification consists mainly in depleting β-O-4 linkages and that guaiacol is enriched in pulp compared to wood (Table 1) points to the 5-5 bond, the second most abundant inter-unit linkage in softwood lignin and more resistant to degradation, as the guaiacol source. On the other hand, 4-methylguaiacol (G 3) was suggested to arise from β-5 structures [27], as proposed by Kuroda and Nakagawa-izumi [42]. The relative increase of 4-methylguaiacol (G 3) in the pulp (Table 1) suggests that it is from a less labile bond, which supports this assumption. Altogether, the products that are less represented in the pulp are possibly arising from β-O-4 linkages, especially so the most affected coniferylaldehyde (G 22), and possibly homovanillin (G 12), coniferyl alcohol (trans) (G 21), eugenol (G 5), and vanillin (G 9).
High homovanillin (G 12) content was associated with more condensed wood lignins (compression wood) [27] and was found to increase during the bleaching of softwood Kraft pulps [19]. However, its decrease from wood to pulp calls for a different and more labile linkage than the one associated with condensed lignin in normal wood. A clearer image could be gained by pulping a wood sample with visible compression wood.
An increase of guaiacol, 4 methylguaiacol, and a decrease of coniferylaldehyde on pulp compared to wood were also reported, although the most important pyrolysis product of the pulps was 4-vinylguaiacol [19,29]. Even though the results cannot be directly compared due to differences in species and analytical pyrolysis instruments (pyrolysis, chromatographic, and detectors).

3.4. The PCA of Pyrolysis Products Separates Samples by Their Lignin Content

About 80% of the variation found on the 25 lignin pyrolysis products among the 30 pulps was captured by PC 1 (69%) and PC 2 (11%). The principal scores plot depicted in Figure 3a shows that pulps are distributed along PC 1 according to their residual lignin content (Figure 3, only Klason lignin content is shown, although identical separation is found for Py-lignin or the kappa number). The loadings plot (Figure 3b) shows that all peaks are in the first and second quadrants; furthermore, the most influential peaks that explain this separation are 4-methylguaiacol (G 3), 4-vinyl guaiacol (G 4), isoeugenol (trans) (G 8), and guaiacol (G 1). The variance explained by PC1 increased to 91% for a PCA using only these 4 variables (result not shown). This means that the pulp samples with higher residual lignin content are comparatively enriched in these pyrolysis products. Interestingly, 4-methylguaiacol (G 3), which accounts for 14.7% of the lignin products on the pulp, and which increased the most from wood to pulp, is the primary pyrolysis product of the pulp lignin (Table 1). As previously stated, 4-methylguaiacol (G 3) is thought to arise from β-5, a less labile C-C condensation inter-unit linkage [27,42]. However, the dihydroconiferyl alcohol (G 19) also found to be associated with 4-methylguaiacol (G 3) [27] is by no means associated with the pulps with higher residual lignin content. On the other hand, high homovanillin (G 12) content was associated with more condensed compression wood lignin [27] and was found to increase during bleaching of softwood Kraft pulps [19]. It is suggested that homovanillin could come from two sources, one more labile than the other. This could explain why homovanillin decreased from wood to pulp, suggesting a more labile bond origin (Table 1), and at the same time increased with increasing residual lignin content, suggesting a less labile origin (Figure 3).

4. Conclusions

This study revealed that analytical pyrolysis can be used to assess the residual lignin content as well as the H/G ratio of maritime pine unbleached Kraft pulps. A good correlation was found between the Py-lignin and lignin content. The average lignin composition (H/G) of the pulps is 2.6 higher than the average lignin composition of the wood, due to a differential removal of G-type lignin during the Kraft process. Analytical pyrolysis combined with PCA can discriminate samples according to lignin content and composition of the pyrolysis products, further providing clues about how lignin is degraded during the Kraft pulping process.

Author Contributions

Conceptualization, A.A., J.G. and J.R.; methodology, A.A.; experimental analysis, A.A.; writing—original draft preparation, A.A., J.G. and J.R.; writing—review and editing, A.A., J.G. and J.R. All authors have read and agreed to the published version of the manuscript.

Funding

FCT: I.P, funded this research via CEF (UIDB/00239/2020) and Ana Alves via contract: DL57/2016/CP1382/CT0005. ADISA was used for the funding of the research and the article processing charge of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Plot of the difference between the lignin content determined by Klason lignin and Py-lignin content against Klason lignin content.
Figure 1. Plot of the difference between the lignin content determined by Klason lignin and Py-lignin content against Klason lignin content.
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Figure 2. Scatterplot of the lignin content determined by pyrolysis versus reference Klason method.
Figure 2. Scatterplot of the lignin content determined by pyrolysis versus reference Klason method.
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Figure 3. Principal scores plot of the PCA using all samples and G- and H-lignin products. (a) Scores plot PC 1 (69% explained variance) versus PC 2 (11% explained variance). Symbols represent 4 arbitrarily defined classes of Klason lignin content. (b) Loadings plot of the G and H-lignin variables (Table 1).
Figure 3. Principal scores plot of the PCA using all samples and G- and H-lignin products. (a) Scores plot PC 1 (69% explained variance) versus PC 2 (11% explained variance). Symbols represent 4 arbitrarily defined classes of Klason lignin content. (b) Loadings plot of the G and H-lignin variables (Table 1).
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Table
Table 1. Distribution of lignin pyrolysis products of maritime pine wood (adapted from [7] and Kraft pulps based on all pyrolysis products (a) and based on lignin products alone (b) (100%).
Table 1. Distribution of lignin pyrolysis products of maritime pine wood (adapted from [7] and Kraft pulps based on all pyrolysis products (a) and based on lignin products alone (b) (100%).
CodeCompoundWood (%a)Pulp (%a)Wood (%b)Pulp (%b)
H 1Phenol0.50.42.26.2
G 1Guaiacol2.20.89.011.1
H 2p-Cresol 0.40.31.53.5
H 3m-Cresol0.30.31.24.5
G 23-Methyl guaiacol0.20.10.61.1
G 34-Methyl guaiacol2.51.110.314.7
G 44-Vinyl guaiacol2.90.912.012.6
G 5Eugenol0.90.13.61.9
G 64-Propyl guaiacol0.10.10.60.7
G 7Isoeugenol (cis)0.20.10.71.4
G 8Isoeugenol (trans)2.00.78.49.7
G 9Vanillin1.90.57.96.4
G 10Indene, 6-hydroxy-7-methoxy-, 1H-1.20.55.26.6
G 11Indene, 6-hydroxy-7-methoxy-, 2H-0.60.22.32.5
G 12Homovanillin1.30.25.42.9
G 13Acetoguaiacone0.90.23.72.7
G 14Guaiacyl acetone0.40.01.80.5
G 15Propioguaiacone0.20.10.90.9
G 16Isomer of coniferyl alcohol0.80.13.32.0
G 17G-CO-CH=CH2 0.50.02.00.6
G 18G-CO-CO-CH30.20.00.90.5
G 19Dihydroconiferyl alcohol0.70.33.14.1
G 20Coniferyl alcohol (cis)0.40.01.50.2
G 21Coniferyl alcohol (trans)0.50.02.00.2
G 22Coniferylaldehyde2.40.29.92.6
Sum247.3100100
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Alves, A.; Graça, J.; Rodrigues, J. Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp. Forests 2022, 13, 2169. https://doi.org/10.3390/f13122169

AMA Style

Alves A, Graça J, Rodrigues J. Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp. Forests. 2022; 13(12):2169. https://doi.org/10.3390/f13122169

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Alves, Ana, José Graça, and José Rodrigues. 2022. "Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp" Forests 13, no. 12: 2169. https://doi.org/10.3390/f13122169

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